The present invention relates to micro-machined electromechanical systems, and more particularly to a MEMS vibratory gyroscope having closed loop output.
Micro-gyroscopes are used in many applications including, but not limited to, communications, control and navigation systems for both space and land applications. These highly specialized applications need high performance and cost effective micro-gyroscopes.
There is known in the art a micro-machined electromechanical vibratory gyroscope designed for micro-spacecraft applications. The gyroscope is explained and described in a technical paper entitled xe2x80x9cSilicon Bulk Micro-machined Vibratory Gyroscopexe2x80x9d presented in June, 1996 at the Solid State Sensors and Actuator Workshop in Hilton Head, S.C.
The prior art gyroscope has a resonator having a xe2x80x9ccloverleafxe2x80x9d structure consisting of a rim, four silicon leaves, and four soft supports, or cantilevers, made from a single crystal silicon. The four supports provide mechanical support and restoring force for the harmonic motion of the structure. A metal baton is rigidly attached to the center of the resonator, in a plane perpendicular to the plane of the silicon leaves, and to a quartz base plate spaced apart from the silicon leaves. The quartz base plate has a pattern of electrodes that coincides with the cloverleaf pattern of the silicon leaves. The electrodes include two drive electrodes and two sense electrodes.
The micro-gyroscope is electrostatically actuated and the sense electrodes capacitively detect Coriolis induced motions of the silicon leaves. The micro-gyroscope has a low resonant frequency due to the large mass of the metal post and the soft cantilevers. The response of the gyroscope is inversely proportional to the resonant frequency. Therefore, a low resonant frequency increases the responsivity of the device.
The cloverleaves provide large areas for electrostatic driving and capacitance sensing. Applying an AC voltage to capacitors that are associated with the drive electrodes excites the resonator. This excites the rotation xe2x8ax96x about the drive axis and rocking-like displacement xe2x8ax96y for the leaves.
Because the post is rigidly attached to the leaves, the rocking movement of the leaves translates to movement of the baton. When the leaves oscillate in the drive mode, the displacement of the post is near parallel to the leaf surface in the y-direction. When a rotation rate is applied about the z-axis, Coriolis force acts on the oscillating post and causes its displacement in the x-direction. The baton displacement is translated back into the rocking motion, xe2x8ax96, of the leaves. The baton provides large Coriolis coupling that transfers energy between the two orthogonal rocking modes.
The control electronics associated with the micro-gyroscope include an actuation circuit that is essentially an oscillator around the micro-gyroscope that locks onto the drive resonance mode. The signals from the sense electrodes are summed to remove the differential signal between them and the response of the sense resonance from the feedback loop. On the other hand, the sense circuit subtracts the signals from the sense electrodes to remove the common-mode drive signal.
Micro-gyroscopes are subject to electrical interference that degrades performance with regard to drift and scale factor stability. Micro-gyroscopes often operate the drive and sense signals at the same frequency to allow for simple electronic circuits. However, the use of a common frequency for both functions allows the relatively powerful drive signal to inadvertently electrically couple to the relatively weak sense signal.
Typically, prior art micro-gyroscopes are open loop and untuned. If the drive frequency is tuned closely to a high Q sense axis resonance, large mechanical gain and low sensitivity to sensor noise is possible. High Q also results in low rate drift.
However, close tuning leads to large uncertainty in the gain and phase of the open-loop response. Phase variations lead to added rate drift errors due to quadrature signal pickup and the gain variations lead to rate scale factor errors. Operating the open-loop micro-gyroscope in a closely tuned manner results in higher scale factor error, higher rate errors due to mechanical phase shifts, and slower response with sensitive lightly damped resonances. Additionally, the response time of the open-loop micro-gyroscope is proportional to the damping time constant, Q, of the sense resonance. To reduce rate drift, very long natural damping time constants are required, slowing the response time.
If the drive frequency is tuned closely to a high Q sense axis resonance, a force-to-rebalance method that incorporates complex demodulators and modulators in multiple re-balance loops is necessary. The modulators and demodulators provide coherent feedback only for signals modulating the drive frequency, and therefore do not provide active damping of independent sense resonance vibrations. These vibrations, if not exactly matched to the drive frequency, are not actively damped resulting in false rate signals or noise.
Noise and drift in the electronic circuit limit micro-gyroscope performance. Therefore, prior art micro-gyroscopes perform poorly and are unreliable in sensitive space applications.
The present invention is a cloverleaf micro-gyroscope having closed loop operation. The differential sense signal (S1xe2x88x92S2) is compensated by a linear electronic filter and directly fed back by differentially changing the voltages on the two drive electrodes (D1xe2x88x92D2) to rebalance Coriolis torque, suppress quadrature motion and increase the damping of the sense axis resonance. The resulting feedback signal is demodulated in phase with the drive axis signal (S1+S2) to produce a measure of the Coriolis force and, hence, the inertial rate input.
The micro-gyroscope of the present invention has a larger sensor bandwidth and is insensitive to tuning errors between drive and sense axis vibration frequency, thereby enabling low-noise tuned operation. The Coriolis force re-balance method of the present invention reduces the complexity of the control circuit and increases the robustness. No modulators or demodulators are necessary in the Coriolis force re-balance loop of the present invention. The presence of external disturbances or tuning errors increases the performance of the micro-gyroscope of the present invention because it provides active damping of the sense axis resonance.
Additionally, rate drift errors (caused by quadrature signal pickup) and rate scale factor errors (caused by gain variations) are reduced in the present invention by the feedback loop gain. Fast response time is possible with lightly damped resonances because the response time in the closed-loop micro-gyroscope of the present invention is proportional to feedback gain and not the natural damping of the resonance.
It is an object of the present invention to reduce the complexity and improve the performance of vibratory micro-gyroscopes. It is another object of the present invention to increase the bandwidth and decrease sensitivity to tuning errors associated with vibratory micro-gyroscopes.
It is a further object of the present invention to provide closed-loop operation of a vibratory micro-gyroscope. It is still a further object of the present invention to compensate the differential sense signal and provide a feedback loop by differentially changing the voltages on the drive electrodes. It is yet a further object of the present invention to demodulate the feedback signal in-phase with the drive axis signal to provide a method of Coriolis force re-balance for a micro-gyroscope.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.